US20030035606A1 - Optical interferometric modulator integrated with optical monitoring mechanism - Google Patents
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- US20030035606A1 US20030035606A1 US10/076,020 US7602002A US2003035606A1 US 20030035606 A1 US20030035606 A1 US 20030035606A1 US 7602002 A US7602002 A US 7602002A US 2003035606 A1 US2003035606 A1 US 2003035606A1
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- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
- G02B6/10—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type
- G02B6/12—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type of the integrated circuit kind
- G02B6/12004—Combinations of two or more optical elements
-
- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
- G02B6/10—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type
- G02B6/12—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type of the integrated circuit kind
- G02B6/122—Basic optical elements, e.g. light-guiding paths
-
- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
- G02B6/24—Coupling light guides
- G02B6/42—Coupling light guides with opto-electronic elements
-
- G—PHYSICS
- G02—OPTICS
- G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
- G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
- G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour
- G02F1/21—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour by interference
- G02F1/225—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour by interference in an optical waveguide structure
-
- G—PHYSICS
- G02—OPTICS
- G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
- G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
- G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour
- G02F1/0121—Operation of devices; Circuit arrangements, not otherwise provided for in this subclass
- G02F1/0123—Circuits for the control or stabilisation of the bias voltage, e.g. automatic bias control [ABC] feedback loops
-
- G—PHYSICS
- G02—OPTICS
- G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
- G02F2201/00—Constructional arrangements not provided for in groups G02F1/00 - G02F7/00
- G02F2201/58—Arrangements comprising a monitoring photodetector
Definitions
- This application relates to optical interferometric modulators for modulating light and devices incorporating such modulators.
- Certain optical interferometric modulators such as Mach-Zehnder electro-optic modulators, modulate the intensity of light based on interference of beams from two optical paths.
- At least one optical path is designed to have an electro-optic material so that a control voltage can be applied to modify the refractive index of the electro-optic material and hence the total optical path length.
- An input optical signal is split into two optical signals that are respectively coupled into the two optical paths.
- the two optical signals undergo different optical path lengths and hence are delayed relative to each other.
- the two optical signals are then spatially combined to interfere with each other to generate an output optical signal.
- the amount of the delay can be adjusted or modulated by the control voltage applied across the electro-optic material.
- the relative delay between the two optical signals is 0, or 2 ⁇ , etc.
- the two signals constructively interfere to produce a maximum intensity output.
- the relative delay is ⁇ , or 3 ⁇ , etc.
- the two signals destructively interfere to produce a minimum intensity output.
- the present disclosure includes techniques for optically monitoring the output optical signals of the above Mach-Zehnder modulators and, more generally, the output optical signals of optical interferometric modulators that use the optical interference between two optical paths to produce an intensity-modulated output optical signal.
- Such optical monitoring uses another optical signal that is ordinarily unused in such an optical modulator and thus does not optically tap the output optical signal of the modulator.
- a device includes an input waveguide, an output waveguide, and first and second waveguides formed on a substrate.
- the first and second waveguides respectively have receiving ends coupled to a port of the input waveguide and output ends coupled to a port of the output waveguide.
- An optical output coupling mechanism is provided to have one end coupled to the output waveguide and another end coupled to an output optical fiber which receives a guided output optical signal from the output waveguide.
- the device also includes an optical detector, displaced from the substrate and positioned near the optical output coupling mechanism, to receive an optical monitor signal that is not guided by either the output waveguide or the output optical fiber. In particular, this unguided optical monitor signal is complementary to the guided output signal.
- An electro-optic material may be used in either or both of the first and the second waveguides to control the difference in the optical path length for the interference operation.
- the unguided optical monitor signal may be used to obtain information that is contained in said guided signal, without directly intercepting the guided signal.
- the unguided optical monitor signal may be used to detect a drift in the optical path length difference between the first and second waveguides with respect to a desired value.
- a feedback control may be used to control the electro-optic material in response to the unguided optical monitor signal to reduce the drift.
- the unguided optical monitor signal may be used to monitor other aspects of the device, such as the bit error rate in the guided output optical signal.
- FIG. 2 shows measured output signals from the optical sampling detector and the output port of the device in FIG. 1.
- FIG. 3 shows a use of an index-matched dielectric material to fill the gap between the optical sampling detector and the output coupling mechanism in the device of FIG. 1.
- FIGS. 5A and 5B show measured signals as functions of positions in one exemplary optical modulator with the sampling detector according to one embodiment.
- FIGS. 7A and 7B show measured signals as a function of time in a modulator based on the design in FIG. 6 for determining whether the sampling detector is set at an optimized location away from the edge of the modulator substrate.
- FIG. 8 shows a flowchart for optimizing the location of the detector based on the signals shown in FIGS. 7A and 7B.
- FIG. 9 shows one implementation of the designs shown in FIG. 6 and FIG. 1.
- the device 100 can turn off and on the guided output signal 108 A to operate as an optical switch or an optical modulator.
- the electro-optic material may be electrically biased at a selected DC voltage.
- An AC control voltage is then applied over the DC bias voltage to control or modulate the guided output signal 108 A.
- the pair of electrodes 122 and 124 may include two separate subsets of electrodes with one subset ( 122 B and 124 B) electrically coupled to provide the DC bias and the other subset ( 122 A and 124 A) electrically coupled to provide the AC control voltage.
- the strength of this unguided portion of the optical energy is complementary to the strength of the guided output signal 108 A.
- any signal variation in the guided signal 108 A such as a change in amplitude, can be faithfully represented by a complementary variation in the unguided portion.
- the output waveguide 108 may be coupled to send the output signal 108 A to an output fiber 142 .
- a facet of a fiber may be directly coupled to the facet of the waveguide 108 on the edge of the substrate 101 to form a waveguide-fiber interface 132 for receiving the guided signal 108 A from the waveguide 108 .
- a waveguide-to-fiber coupler may be used at the interface 132 to facilitate the waveguide-fiber coupling.
- such a coupler may be a pigtail fiber coupler.
- a fiber-to-waveguide interface 130 may also be formed to couple an input fiber 140 to the input waveguide 102 , e.g., by either directly coupling or using a coupler device.
- the detector 150 may be preferably placed above the interface 132 where the signal strength of the unguided portion is generally higher than other locations along the output waveguide 108 .
- FIG. 2 shows measured output optical signal 108 A and the output 152 of the detector 150 , where the unguided output 152 is complementary to the guided signal 108 A in time domain: the signal 152 increases when the signal 108 A decreases, reaches a maximum amplitude when the signal 108 A reaches a minimum amplitude, and vice versa.
- the output signal 152 of the detector 150 may be used to monitor the performance of the optical modulator 100 .
- the modulator may be used to superimpose digital data on the input signal 102 A in the waveguide 120 as an optical carrier by applying modulated voltages representing the data across the electrodes 122 A and 124 A.
- the electrodes 122 B and 124 B are biased at a desired DC voltage.
- a data circuit 410 is used to provide an AC modulation voltage 412 that represents the data.
- the output signal 152 of the detector 150 may be used to monitor the bit error rate of the output signal 108 A.
- the output signal 152 of the detector 150 may also be used to determine whether the DC bias voltage across the bias electrodes 122 B and 124 is at or near a DC value so that the DC value of the optical path length difference between the waveguides 110 and 120 at the coupler 106 is biased at a desired value.
- the DC bias point may be selected to operate the device within a particular linear range, at a minimal zero throughput, or at a half power point of the optical output.
- the bias voltage may be applied to the some or all of the electrodes that apply the AC fields.
- the DC bias value of the optical path length difference between the waveguides 110 and 120 at the coupler 106 may drift from the desired bias value due to a number of factors.
- the temperature may vary and hence the physical length and the refractive index of each waveguide may change with the temperature.
- U.S. Pat. No. 6,181,456 B1 to McBrien et al. describes other common factors that contribute to the bias drift.
- the actual electric field applied to the electro-optic portion of the waveguide may vary to cause the bias point of the device to drift. Physical impurities, crystal defects, and any causes of both trapped and mobile charges may affect the bias stability of the device.
- FIG. 4 shows an exemplary electro-optic modulator 400 that uses one active bias feedback control mechanism for reducing the bias drift.
- the optical detector 150 as described above is used to receive the unguided optical energy emanated from the waveguide-fiber interface 132 and to produce the detector output 152 that is complementary to the guided signal 108 A in the output waveguide 108 .
- a bias feedback control unit 420 measures the DC level of the signal 152 which is correlated to the DC level of the guided output signal 108 A. Based on this measurement, the control unit 420 determines the bias drift and produces a control signal 422 that adjusts the DC voltage on the electrodes 122 B and 124 B to reduce the bias drift.
- the unguided signal complementary to the guided signal in the output of the modulator 100 in FIG. 1 or 400 in FIG. 4 varies with position.
- a xyz coordinate system is shown to have its x axis perpendicular to the output waveguide 108 and parallel to the surface plane of the substrate 101 , the z axis perpendicular to the waveguide the surface plane of the supporting substrate 101 , and the y axis parallel to the output waveguide 108 .
- the origin of the xyz coordinate system is assumed to be at the interfacing point between the output waveguide 108 and the output fiber 142 on the substrate surface.
- the output fiber 142 also has its axis along the y axis.
- FIG. 5A shows the variation of the unguided complementary signal with respect to the z positions above the substrate surface for a given y (>0) location.
- the signal generally decays with z.
- FIG. 5B further shows the y dependence of the unguided complementary signal at a given z value.
- the signal is approximately at its maximum right above the interface between the output waveguide 108 and the output fiber 142 and decays along both the +y direction along the output fiber 142 and ⁇ y direction along the output waveguide 108 towards the joint 106 of the two waveguides 110 and 120 .
- FIG. 5B shows that the unguided complementary signal decays more rapidly along the ⁇ y direction than the +y direction.
- the detector 150 may generally be located above the interfacing location near the waveguide-fiber interface 132 (y ⁇ 0) or above the output fiber 142 (y>0) to achieve a high detection sensitivity.
- the detector 150 is shown to be located approximately above the interfacing point between the output waveguide 108 and the output fiber 142 where the unguided complementary signal is approximately at its maximum.
- the guided signal encounters a transition between two different optical media from the waveguide 108 to a different material such as the output fiber 142 .
- a portion of the guided signal generally scatters at the exit facet of the output waveguide.
- the scattered light from the guided light may no longer be in the guide mode and thus may mix with the unguided complementary signal.
- the detector 150 when the detector 150 is located at or sufficiently close to the exit facet of the waveguide 108 at the edge of the substrate 101 , it may receive the scattered light caused by the scattering of the guided light. This received scattered light is not complementary to the guided signal and therefore contributes noise to the output of the detector 150 which is to detect the unguided complementary signal.
- FIG. 8 shows the steps for placing the detector 150 at the optimized position y o according to one embodiment.
- FIG. 9 shows one exemplary implementation 900 of an optical Mach-Zehnder modulator 910 with an optical sampling detector 150 based on the design in FIG. 6.
- the modulator device 900 includes a modulator housing 902 that encloses the optical modulator 910 and the optical detector 150 with one end optically coupled to an input fiber 962 for receiving an input optical signal and another end optically coupled to an output fiber 952 for exporting a modulated output signal.
- the modulator 910 may be formed on an electro-optic crystal substrate such as a lithium Niobate or other substrates exhibiting electro-optic effects.
- Circuits 912 are engaged to the modulator housing 902 with a feedthrough design and are electrically coupled to the modulator 910 to provide electrical bias and electrical modulation control signal.
- the housing 902 may be hermetically sealed.
- Two fiber support ks 930 and 940 are respectively formed on two ends of the lator 910 to engage output fiber 904 and input fiber 903 he modulator 910 .
- the fiber 904 is engaged to the housing through a fiber fitting unit 950 mounted on the end of the ing 902 and extends outside the housing 902 as the fiber which may be generally covered with the fiber protection er material.
- a fiber fitting unit 960 is ted on the opposite end of the housing 902 to engage the t fiber 903 to the housing 902 .
- the portion of the input r 903 outside the housing 902 is indicated by the numeral which may be generally covered with the fiber protection er material.
- the t fiber 903 may be a polarization maintaining fiber and output fiber 904 may be a single-mode fiber.
- the optical ctor 150 may be mounted to a detector mounting block 920 h has the electrical connection for the detector 150 .
- a through port 922 may be formed on the housing 902 to ide an electrical conduit to the electrical connection of detector 150 .
- FIGS. 11A, 11B, and 11 C show additional details of the detector 150 and its mounting mechanism.
- the detector mounting block 920 is shown to have a horseshoe design where an opening 110 is formed to receive and hold the detector 150 .
- the top surface of the detector mounting block 920 has anode and cathode electrodes 1110 and 1120 that are separated from one another by a gap 1130 .
- the detector mounting block 920 may be formed from a ceramic material coated with a conductive film.
- the conductive film is patterned to form the electrodes 1110 and 1120 .
- the detector 150 is electrically coupled to the electrodes 1110 and 1120 .
- FIG. 11B shows an exploded view of the detector mounting mechanism where a cover 1040 for the housing 902 is also shown.
- the material of the substrate 101 and the material for the fibers 903 and 904 may be different and thus have different coefficients of thermal expansion.
- An interface between the substrate 101 and the fiber hence, may be subject to an axial stress along the fiber due to a variation in temperature. This axial stress is undesirable because it may cause misalignment between the waveguide in the substrate 101 and the fiber and hence cause unwanted optical loss.
- the housing 902 in which the modulator 920 is mounted may also be formed of a material (e.g., a metal) different from the substrate 101 . This may cause additional thermal stresses.
- Table I lists the coefficients of thermal expansion of different materials that may be used in the above modulator devices where a metallic alloy such as Kovar may be used to construct the housing 902 and a metallic alloy Invar may be used as inserts between dissimilar materials to reduce the overall thermal expansion as discussed below.
- a metallic alloy such as Kovar
- Invar metallic alloy
- One aspect of this application is to provide an athermal design for the waveguide-to-fiber interface to reduce thermal stresses when the unit experiences a variation in temperature.
- the athermal design may be achieved by selecting materials with different coefficients of thermal expansion to reduce the net thermal effect at one or more selected locations, e.g., the interface between the waveguide and the fiber.
- FIG. 12 shows one embodiment 1200 of an athermal design in which the lithium niobate crystal 1210 is bonded to the modulator housing 1220 formed of the alloy Kovar. End caps 1230 and 1260 are engaged to the housing 1220 for holding the input fiber 1240 and output fiber 1250 , respectively.
- the athermal design for the fiber to crystal attachment is to set the following to zero:
- FIG. 14 shows some assembly details of the above design.
- the Kovar housing is preassembled with glass beaded feedthrus and case grounding pins for subsequent attachment of the electrical connections to the crystal to provide access to a printed circuit board assembly.
- the input end cap with the brazed in Kovar ferrule is brazed to the housing.
- the preassembled crystal-and-fiber assembly may be inserted from the open end of the housing assembly.
- the input PMF fiber is first inserted and threaded through the input Invar ferrule.
- the crystal is then positioned and bonded to the bottom of the housing with a compliant adhesive in the central portion of the bottom of the crystal and the Kovar housing. Portions of the Kovar housing are in contact with the crystal outside of the bond joint.
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Abstract
Description
- This application is a continuation-in-part application of U.S. application Ser. No. 09/797,783 entitled “OPTICAL MONITORING IN OPTICAL INTERFEROMETRIC MODULATORS” and filed Mar. 1, 2001 which claims the benefit of U.S. Provisional Application No. 60/260,581 filed Jan. 9, 2001. This application further claims benefits of U.S. Provisional Application No. 60/268,430 filed Feb. 12, 2001 and 60/274,131 filed Mar. 7, 2001. The disclosures of the above-related applications are incorporated herein by reference as part of the specification of this application.
- This application relates to optical interferometric modulators for modulating light and devices incorporating such modulators.
- Certain optical interferometric modulators, such as Mach-Zehnder electro-optic modulators, modulate the intensity of light based on interference of beams from two optical paths. At least one optical path is designed to have an electro-optic material so that a control voltage can be applied to modify the refractive index of the electro-optic material and hence the total optical path length. An input optical signal is split into two optical signals that are respectively coupled into the two optical paths. The two optical signals undergo different optical path lengths and hence are delayed relative to each other. The two optical signals are then spatially combined to interfere with each other to generate an output optical signal.
- The amount of the delay can be adjusted or modulated by the control voltage applied across the electro-optic material. Hence, when the relative delay between the two optical signals is 0, or 2π, etc., the two signals constructively interfere to produce a maximum intensity output. However, when the relative delay is π, or 3π, etc., the two signals destructively interfere to produce a minimum intensity output.
- The present disclosure includes techniques for optically monitoring the output optical signals of the above Mach-Zehnder modulators and, more generally, the output optical signals of optical interferometric modulators that use the optical interference between two optical paths to produce an intensity-modulated output optical signal. Such optical monitoring uses another optical signal that is ordinarily unused in such an optical modulator and thus does not optically tap the output optical signal of the modulator.
- A device according to one embodiment includes an input waveguide, an output waveguide, and first and second waveguides formed on a substrate. The first and second waveguides respectively have receiving ends coupled to a port of the input waveguide and output ends coupled to a port of the output waveguide. An optical output coupling mechanism is provided to have one end coupled to the output waveguide and another end coupled to an output optical fiber which receives a guided output optical signal from the output waveguide. The device also includes an optical detector, displaced from the substrate and positioned near the optical output coupling mechanism, to receive an optical monitor signal that is not guided by either the output waveguide or the output optical fiber. In particular, this unguided optical monitor signal is complementary to the guided output signal.
- An electro-optic material may be used in either or both of the first and the second waveguides to control the difference in the optical path length for the interference operation. The unguided optical monitor signal may be used to obtain information that is contained in said guided signal, without directly intercepting the guided signal. For example, the unguided optical monitor signal may be used to detect a drift in the optical path length difference between the first and second waveguides with respect to a desired value. A feedback control may be used to control the electro-optic material in response to the unguided optical monitor signal to reduce the drift. In another example, the unguided optical monitor signal may be used to monitor other aspects of the device, such as the bit error rate in the guided output optical signal.
- FIG. 1 illustrates a Mach-Zehnder electro-optic modulator with an optical sampling detector according to one embodiment.
- FIG. 2 shows measured output signals from the optical sampling detector and the output port of the device in FIG. 1.
- FIG. 3 shows a use of an index-matched dielectric material to fill the gap between the optical sampling detector and the output coupling mechanism in the device of FIG. 1.
- FIG. 4 shows a bias feedback control based on the output of the optical sampling detector in FIG. 1.
- FIGS. 5A and 5B show measured signals as functions of positions in one exemplary optical modulator with the sampling detector according to one embodiment.
- FIG. 6 shows one embodiment of an optical modulator with the sampling detector located away from the edge of the modulator substrate.
- FIGS. 7A and 7B show measured signals as a function of time in a modulator based on the design in FIG. 6 for determining whether the sampling detector is set at an optimized location away from the edge of the modulator substrate.
- FIG. 8 shows a flowchart for optimizing the location of the detector based on the signals shown in FIGS. 7A and 7B.
- FIG. 9 shows one implementation of the designs shown in FIG. 6 and FIG. 1.
- FIGS. 10, 11A,11B, and 11C show mounting of the sampling detector for the implementation of FIG. 9 based on the design in FIG. 6.
- FIGS. 12 and 13 show athermal designs of the modulator along axial and radial directions of the fibers, respectively.
- FIG. 14 shows assembly of an end cap to a modulator housing according to one embodiment.
- FIG. 1 shows a Mach-Zehnder electro-
optic modulator 100 as an example for a general optical interferometric modulator to illustrate the optical monitoring technique according to one embodiment. Themodulator 100 is formed over asubstrate 101. Opticaldielectric waveguides substrate 101. Thewaveguide 102 is the input waveguide to receive an inputoptical signal 102A. Thewaveguide 108 is the output waveguide to output an outputoptical signal 108A. Thewaveguides - An
optical waveguide coupler 104 is formed as an input port to couple receiving ends of thewaveguides input waveguide 102. Theinput signal 102A in theinput waveguide 102 is thus split by thecoupler 104 into afirst signal 110A in thewaveguide 110 and asecond signal 120A in thewaveguide 120. At least a portion of thewaveguide 120 includes an electro-optic material whose refractive index changes in response to a control voltage.Electrodes waveguide 120 to supply this control voltage. The change in the refractive index of the electro-optical material changes the total optical path length (i.e., a sum of the products of the index and the physical length of all segments in the path) of thewaveguide 120. This change can be used to control the difference in the optical path length of the twowaveguides waveguides - Another
optical waveguide coupler 106 is also formed on thesubstrate 101 as an output port to join the output ends of thewaveguides output waveguide 108. Hence, the twooptical signals coupler 106 to produce theoutput signal 108A. Notably, theoutput signal 108A is optically guided by theoutput waveguide 108. Theoutput signal 108A has a maximum amplitude when the total difference in optical path lengths of the twosignals coupler 106 is equal to Kλ (i.e., a constructive interference) and has a minimum amplitude when the difference is (2K+1)λ (i.e., a destructive interference), where K=0, ±1, ±2, . . . . In terms of the amount of the relative delay between the twooptical signals output 108A is produced when the delay is 0, or 2π, etc., and the minimum amplitude in theoutput signal 108A is produced when the relative delay is π, or 3π, etc. - Hence, by controlling the voltage on the
electrodes device 100 can turn off and on the guidedoutput signal 108A to operate as an optical switch or an optical modulator. In general, the electro-optic material may be electrically biased at a selected DC voltage. An AC control voltage is then applied over the DC bias voltage to control or modulate the guidedoutput signal 108A. Referring to FIG. 4, the pair ofelectrodes - It is recognized that, in addition to the above guided output
optical signal 108A in the guided mode, a portion of the optical energy produced by the optical interference at thecoupler 106 also dissipates outside theoutput waveguide 108 in an unguided mode into the surrounding areas of thecoupler 106, including thesubstrate 101. Under the conservation of energy at thecoupler 106, the total sum of this unguided portion and the guidedportion 108 is equal to the total sum of the receivedsignals 110A and 110B by thecoupler 106. Hence, when the interference is constructive, theoutput signal 108A reaches its maximum while the unguided portion is at its minimum. Conversely, when the interference is destructive, theoutput signal 108A reaches its minimum while the unguided portion is at its maximum. Therefore, the strength of this unguided portion of the optical energy is complementary to the strength of the guidedoutput signal 108A. As a result, any signal variation in the guidedsignal 108A, such as a change in amplitude, can be faithfully represented by a complementary variation in the unguided portion. - In particular, this unguided portion of optical energy can be collected and measured without optically affecting the guided
signal 108A. This is because this unguided portion of optical energy is not guided by theoutput waveguide 108 and hence there is no need to optically tap theoutput signal 108A either at theoutput waveguide 108 or somewhere in the downstream of theoutput waveguide 108 in order to monitor the guidedsignal 108A. The unguided portion can be collected at a location where the unguided portion of energy is present. - For example, as illustrated in FIG. 1, an
optical detector 150 may be positioned above theoutput waveguide 108 over thesubstrate 101 to receive the unguided portion and to produce anoptical monitoring signal 152 that is complementary to theoutput signal 108A. Notably, theoptical detector 150 need not be optically coupled to receive the guidedoutput signal 108A and therefore the presence and operation of thedetector 150 do not interfere with any aspect of transmission and subsequent processing of theoutput signal 108A. A lens, a lens combination, or other suitable optical collecting element, may be positioned between thesubstrate 101 and thedetector 150 to increase the effective collecting aperture of thedetector 150. - It is further recognized that, the unguided portion of the optical energy in the
substrate 101 emanates outside the substrate at the boundary of thesubstrate 101 near theoutput waveguide 108. Hence, thedetector 150 may be positioned above the output end of theoutput waveguide 108 at the edge of thesubstrate 101 to collect the unguided portion. - In applications where optical fibers are used, the
output waveguide 108 may be coupled to send theoutput signal 108A to anoutput fiber 142. A facet of a fiber may be directly coupled to the facet of thewaveguide 108 on the edge of thesubstrate 101 to form a waveguide-fiber interface 132 for receiving the guidedsignal 108A from thewaveguide 108. Alternatively, a waveguide-to-fiber coupler may be used at theinterface 132 to facilitate the waveguide-fiber coupling. For example, such a coupler may be a pigtail fiber coupler. At the input side of thedevice 100, a fiber-to-waveguide interface 130 may also be formed to couple aninput fiber 140 to theinput waveguide 102, e.g., by either directly coupling or using a coupler device. In this configuration, thedetector 150 may be preferably placed above theinterface 132 where the signal strength of the unguided portion is generally higher than other locations along theoutput waveguide 108. FIG. 2 shows measured outputoptical signal 108A and theoutput 152 of thedetector 150, where theunguided output 152 is complementary to the guidedsignal 108A in time domain: thesignal 152 increases when thesignal 108A decreases, reaches a maximum amplitude when thesignal 108A reaches a minimum amplitude, and vice versa. - FIG. 3 shows that, a
transparent dielectric block 310 may be formed in the air gap between thedetector 150 and the waveguide-fiber interface 132 to enhance the collection efficiency of thedetector 150. The refractive index of theblock 310 may approximately match that of thefiber 142. In this configuration, the unguided portion in thesubstrate 101 propagates along theoutput waveguide 108 and is emanated outside thesubstrate 101 at the waveguide-fiber interface 132 where thesubstrate 101 discontinues. A sufficient portion of the emanated energy is collected into theblock 310 to thedetector 150. - Alternatively, the
optical detector 150 may be formed in thesubstrate 101 near theoutput waveguide 108 to collect the unguided portion dissipating into thesubstrate 101. - The
output signal 152 of thedetector 150 may be used to monitor the performance of theoptical modulator 100. As the example shown in FIG. 4, the modulator may be used to superimpose digital data on theinput signal 102A in thewaveguide 120 as an optical carrier by applying modulated voltages representing the data across theelectrodes electrodes data circuit 410 is used to provide anAC modulation voltage 412 that represents the data. Theoutput signal 152 of thedetector 150 may be used to monitor the bit error rate of theoutput signal 108A. - As another example, the
output signal 152 of thedetector 150 may also be used to determine whether the DC bias voltage across thebias electrodes waveguides coupler 106 is biased at a desired value. The DC bias point may be selected to operate the device within a particular linear range, at a minimal zero throughput, or at a half power point of the optical output. In some instances, the bias voltage may be applied to the some or all of the electrodes that apply the AC fields. - However, the DC bias value of the optical path length difference between the
waveguides coupler 106 may drift from the desired bias value due to a number of factors. For example, the temperature may vary and hence the physical length and the refractive index of each waveguide may change with the temperature. U.S. Pat. No. 6,181,456 B1 to McBrien et al. describes other common factors that contribute to the bias drift. For example, although a constant DC bias voltage is applied, the actual electric field applied to the electro-optic portion of the waveguide may vary to cause the bias point of the device to drift. Physical impurities, crystal defects, and any causes of both trapped and mobile charges may affect the bias stability of the device. In addition, because the optical waveguides are typically located near the surface of the electro-optic substrate, the crystal composition near the surface affects drift of the bias point via a variety of surface chemistry mechanisms. Even the method used for fabricating the waveguides, often involving infusion or proton exchange processes, may affect the bias point drift, because these techniques generally modify the crystal structure. Such bias drift may adversely affect the performance of the device and hence it is desirable to reduce the drift. - FIG. 4 shows an exemplary electro-
optic modulator 400 that uses one active bias feedback control mechanism for reducing the bias drift. Theoptical detector 150 as described above is used to receive the unguided optical energy emanated from the waveguide-fiber interface 132 and to produce thedetector output 152 that is complementary to the guidedsignal 108A in theoutput waveguide 108. A biasfeedback control unit 420 measures the DC level of thesignal 152 which is correlated to the DC level of the guidedoutput signal 108A. Based on this measurement, thecontrol unit 420 determines the bias drift and produces acontrol signal 422 that adjusts the DC voltage on theelectrodes - Notably, the unguided signal complementary to the guided signal in the output of the
modulator 100 in FIG. 1 or 400 in FIG. 4 varies with position. Referring back to FIG. 1, a xyz coordinate system is shown to have its x axis perpendicular to theoutput waveguide 108 and parallel to the surface plane of thesubstrate 101, the z axis perpendicular to the waveguide the surface plane of the supportingsubstrate 101, and the y axis parallel to theoutput waveguide 108. In the following description, the origin of the xyz coordinate system is assumed to be at the interfacing point between theoutput waveguide 108 and theoutput fiber 142 on the substrate surface. Theoutput fiber 142 also has its axis along the y axis. Thedetector 150 may be generally located at positions with x=0 and properly selected y and z positions. - FIG. 5A shows the variation of the unguided complementary signal with respect to the z positions above the substrate surface for a given y (>0) location. The signal strength is expressed in terms of the percentage change from the maximum signal strength measured at z=0. The signal generally decays with z. FIG. 5B further shows the y dependence of the unguided complementary signal at a given z value. The signal is approximately at its maximum right above the interface between the
output waveguide 108 and theoutput fiber 142 and decays along both the +y direction along theoutput fiber 142 and −y direction along theoutput waveguide 108 towards the joint 106 of the twowaveguides - In particular, FIG. 5B shows that the unguided complementary signal decays more rapidly along the −y direction than the +y direction. Hence, the
detector 150 may generally be located above the interfacing location near the waveguide-fiber interface 132 (y≈0) or above the output fiber 142 (y>0) to achieve a high detection sensitivity. In the embodiment in FIG. 3, for example, thedetector 150 is shown to be located approximately above the interfacing point between theoutput waveguide 108 and theoutput fiber 142 where the unguided complementary signal is approximately at its maximum. - At the exit facet of the
output waveguide 108 at the edge of thesubstrate 101, however, the guided signal encounters a transition between two different optical media from thewaveguide 108 to a different material such as theoutput fiber 142. In absence of any index-matching mechanism, a portion of the guided signal generally scatters at the exit facet of the output waveguide. The scattered light from the guided light may no longer be in the guide mode and thus may mix with the unguided complementary signal. Hence, when thedetector 150 is located at or sufficiently close to the exit facet of thewaveguide 108 at the edge of thesubstrate 101, it may receive the scattered light caused by the scattering of the guided light. This received scattered light is not complementary to the guided signal and therefore contributes noise to the output of thedetector 150 which is to detect the unguided complementary signal. - This noise caused by the scattering of the unguided light at the
interface 132, however, decays significantly with the distance from theinterface 132. In particular, the spatial decay of this noise is faster than the decay of the unguided complementary signal outside both theoutput waveguide 108 and theoutput fiber 142. As shown in FIG. 5B, the signal strength of the unguided complementary signal at y>0 decreases from its maximum value at the interface y≈0. However, FIG. 5B also indicates that this decrease is gradual, e.g., approximately a few percent (less than 3%) over a range of at least 700 microns from theinterface 132 above theoutput fiber 142. Measurements show that, thedetector 150 may be situated away from theinterface 132 to locate at a selected location above theoutput fiber 142 where z=zs>0, y=ys>0, and x≈0 to reduce the amount of the scattered guided light received by thedetector 150. At this location, thedetector 150 still receives a significant amount of the unguided complementary signal to achieve an acceptable signal to noise ratio. - The
detector 150 may be generally placed above the output fiber with zs within a few hundred microns (e.g., less than 100 microns) above theoutput fiber 142. The y position away from theinterface 132, ys, may be generally selected by maintaining the signal to noise ratio above an acceptable minimum value. - In one implementation, the y position of the
detector 152, ys, may be selected at an optimized yo by directly observing the amplitude variation of theoutput signal 152 of thedetector 150. This is based on the discovery that, at a given zs value and x=0, the modulated signal peaks with respect to time in theoutput signal 152 have substantially the same amplitude when the y position is at this optimized position yo. When the position ys of thedetector 150 deviates from this optimized position, the amplitudes of two adjacent peaks become different. - FIGS. 7A and 7B are measurements of a Mach-Zehnder modulator with a sampling detector based on the design in FIG. 6. The top trace in each figure is the measured signal representing the guided
output light 108A in theoutput waveguide 108 which is coupled into theoutput fiber 142. The lower trace in each figure, on the other hand, represents the measureddetector output 152 from thedetector 150 that represents the unguided light emanating out of thewaveguide 108. The lower trace is phase shifted from the upper trace to be complementary in time. FIG. 7A shows the measureddetector signal 152 when thedetector 150 is at the optimized position yo. FIG. 7B shows the measureddetector signal 152 when thedetector 150 is away from the optimized position yo where a modulation peak has a different amplitude with the immediate adjacent modulation peak but the same amplitude with the next adjacent modulation peak. - FIG. 8 shows the steps for placing the
detector 150 at the optimized position yo according to one embodiment. First, thedetector 150 is situated above the fiber 142 (x=0) with a selected height zs. Then thedetector 150 is adjusted along the y direction to be away from the edge of theinterface 132 to a position where the peak signals of thedetector output 152 are substantially equal. Finally, the position of thedetector 150 is fixed at the optimized position yo. - FIG. 9 shows one
exemplary implementation 900 of an optical Mach-Zehnder modulator 910 with anoptical sampling detector 150 based on the design in FIG. 6. Themodulator device 900 includes amodulator housing 902 that encloses theoptical modulator 910 and theoptical detector 150 with one end optically coupled to aninput fiber 962 for receiving an input optical signal and another end optically coupled to anoutput fiber 952 for exporting a modulated output signal. Themodulator 910 may be formed on an electro-optic crystal substrate such as a lithium Niobate or other substrates exhibiting electro-optic effects.Circuits 912 are engaged to themodulator housing 902 with a feedthrough design and are electrically coupled to themodulator 910 to provide electrical bias and electrical modulation control signal. Thehousing 902 may be hermetically sealed. Twofiber support ks output fiber 904 andinput fiber 903 he modulator 910. Thefiber 904 is engaged to the housing through a fiberfitting unit 950 mounted on the end of theing 902 and extends outside thehousing 902 as the fiber which may be generally covered with the fiber protection er material. Similarly, a fiberfitting unit 960 is ted on the opposite end of thehousing 902 to engage thet fiber 903 to thehousing 902. The portion of theinput r 903 outside thehousing 902 is indicated by the numeral which may be generally covered with the fiber protection er material. An additional elastomer strain release unit be placed over the fitting 950 to protect the fiber. Thet fiber 903 may be a polarization maintaining fiber andoutput fiber 904 may be a single-mode fiber. Theoptical ctor 150 may be mounted to a detector mounting block 920 h has the electrical connection for thedetector 150. A throughport 922 may be formed on thehousing 902 to ide an electrical conduit to the electrical connection ofdetector 150. - FIG. 10 shows the portion B of the modulator device in a sectional view along the direction A-A. In this diment, the
crystal 1010 is an electro-optic material and s the substrate for themodulator 910 on which the waveguides for the Mach-Zehnder modulator are fabricated. As illustrated, thefiber 904 is held by thefiber support block 930 to have its receiving end facet against the output facet of the output waveguide at the edge of thecrystal 1010. Thefiber support block 930 may be formed of a transparent material, e.g., such as a glass, to allow transmission of the unguided complementary signal to thedetector 150. Thedetector mounting block 920 may be mounted on top of thefiber support block 930 or an inner part of thehousing 902 to place thedetector 150 at a selected position ys above thefiber 904 and away from the edge of thecrystal 1010. FIG. 10 shows the implementation in which the interior of thehousing 902 hasplatforms 1030 at two opposite sides of thefiber 904 to support thedetector mounting block 920 above thefiber 904 and thefiber support block 930. - FIGS. 11A, 11B, and11C show additional details of the
detector 150 and its mounting mechanism. In FIG. 11A, thedetector mounting block 920 is shown to have a horseshoe design where anopening 110 is formed to receive and hold thedetector 150. The top surface of thedetector mounting block 920 has anode andcathode electrodes gap 1130. In one implementation, thedetector mounting block 920 may be formed from a ceramic material coated with a conductive film. The conductive film is patterned to form theelectrodes detector 150 is electrically coupled to theelectrodes cover 1040 for thehousing 902 is also shown. - FIG. 11C shows the electrical connections for the
detector 150. The electrical feedthrough 112 has two conductors with their ends inside thehousing 902 respectively connected to theelectrodes conductive wires 1160. The electrical connections between thedetector 150 and theelectrodes separate contact locations - It is recognized that, the material of the
substrate 101 and the material for thefibers substrate 101 and the fiber, hence, may be subject to an axial stress along the fiber due to a variation in temperature. This axial stress is undesirable because it may cause misalignment between the waveguide in thesubstrate 101 and the fiber and hence cause unwanted optical loss. In addition, thehousing 902 in which themodulator 920 is mounted may also be formed of a material (e.g., a metal) different from thesubstrate 101. This may cause additional thermal stresses. Table I lists the coefficients of thermal expansion of different materials that may be used in the above modulator devices where a metallic alloy such as Kovar may be used to construct thehousing 902 and a metallic alloy Invar may be used as inserts between dissimilar materials to reduce the overall thermal expansion as discussed below.TABLE I COEFFICIENT OF THERMAL EXPANSION MATERIAL C(PPM/° C.) Lithium 17.9 Niobate (modulator) Kovar 5.5 (housing) Glass Fiber 0.8 Copper (end 17.6 caps) Invar 1.2 (insert) - One aspect of this application is to provide an athermal design for the waveguide-to-fiber interface to reduce thermal stresses when the unit experiences a variation in temperature. The athermal design may be achieved by selecting materials with different coefficients of thermal expansion to reduce the net thermal effect at one or more selected locations, e.g., the interface between the waveguide and the fiber.
- FIG. 12 shows one
embodiment 1200 of an athermal design in which thelithium niobate crystal 1210 is bonded to themodulator housing 1220 formed of the alloy Kovar. End caps 1230 and 1260 are engaged to thehousing 1220 for holding theinput fiber 1240 andoutput fiber 1250, respectively. The athermal design for the fiber to crystal attachment is to set the following to zero: - C(output fiber)L(input fiber)+C(crystal)L(crystal)+C(input fiber)L(output fiber)−[C(input end cap)L(input end cap)+C(housing)L(housing)+C(output end cap)L(output end cap)]
- where C represents the coefficient of thermal expansion of each component and L the length of each component. In implementation, the materials and the lengths of the components are selected to make the total sum substantially zero. In this example, the end caps are made of copper to achieve a large amount of thermal expansion and the housing is made of Kovar to achieve a small amount of thermal expansion in order to satisfy the above athermal design.
- The above athermal design is to reduce the axial thermal expansion along the fiber's longitudinal direction. The thermal stress along the radial direction may also be adverse to the modulator module because such stress may cause misalignment. In addition, the redial stress exerted on the fiber at the end of the end caps may change the polarization property of the PM fibers.
- FIG. 13 shows one
embodiment 1300 of the engagement of theend cap 1230 and thefiber 1240. As described above, theend cap 1230 may be formed of a metal such as copper with a large coefficient of thermal expansion to meet the athermal design in the axial direction. Under this design, the coefficients of thermal expansion of the fiber and the end caps are large. Hence, the thermal-induced stress along the radial direction is large and is undesirable. To reduce this radial thermal stress, aninsert member 1310 is inserted between thecopper end cap 1230 and thefiber 1240. The thermal expansion of theinsert member 1310 is selected to be close to that of the fiber glass and is smaller than that of theend cap 1230. In particular, the radial dimension of theinsert member 1310 is made to be greater than that of theend cap 1230 to dominate the radial dimension to reduce the effect of the radial strain caused by theend cap 1230. Theinsert member 1310 may be formed of Invar whose CTE of 1.2 PPM/° C. closely matches that of the glass fiber (0.8 PPM/° C.). The Invar insert may be brazed or press fitted into thecopper end cap 1230 prior to assembly. The surface of thefiber 1240 may be metalized and a low-temperatureindium alloy solder 1320 may be used to seal the interface between the metalized fiber and the Invar insert member. In this design, the effect of the dimensional variance of the copper end cap on the fiber is substantially reduced. - FIG. 14 shows some assembly details of the above design. The Kovar housing is preassembled with glass beaded feedthrus and case grounding pins for subsequent attachment of the electrical connections to the crystal to provide access to a printed circuit board assembly. The input end cap with the brazed in Kovar ferrule is brazed to the housing. Next, the preassembled crystal-and-fiber assembly may be inserted from the open end of the housing assembly. The input PMF fiber is first inserted and threaded through the input Invar ferrule. The crystal is then positioned and bonded to the bottom of the housing with a compliant adhesive in the central portion of the bottom of the crystal and the Kovar housing. Portions of the Kovar housing are in contact with the crystal outside of the bond joint. Prior to introducing the crystal, or possibly subsequent to its bonding in the housing, the exit ferrule and end cap is slid down the exit fiber. The end cap is brought up to the Kovar housing and soldered with a low-temperature solder pre-form material by using, e.g., a heated gas heat source. After the end cap is hermetically sealed by means of the solder to the housing, the Invar ferrules are solder sealed around the metalized fiber. A buffer is slid along the exit SMF fiber and epoxy bonded to the Invar ferrule. Electrical connections may be made from the lithium niobate crystal to the feedthrus or to a housing ground. After functional hookup of the crystal is achieved, the photo detector is installed at the desired location as described above. After all internal assembly operations are accomplished, the housing cover is put in place and seam welded by using established manufacturing assembly processes to effect a hermetically sealed assembly.
- Although the present disclosure only describes a few embodiments, it is understood that various modifications and enhancements may be made without departing from the following claims.
Claims (18)
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US10/076,020 US6647185B2 (en) | 2001-01-09 | 2002-02-12 | Optical interferometric modulator integrated with optical monitoring mechanism |
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US26058101P | 2001-01-09 | 2001-01-09 | |
US26843001P | 2001-02-12 | 2001-02-12 | |
US09/797,783 US6421483B1 (en) | 2001-01-09 | 2001-03-01 | Optical monitoring in optical interferometric modulators |
US27413101P | 2001-03-07 | 2001-03-07 | |
US10/076,020 US6647185B2 (en) | 2001-01-09 | 2002-02-12 | Optical interferometric modulator integrated with optical monitoring mechanism |
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WO2008137204A1 (en) * | 2007-03-20 | 2008-11-13 | Massachusetts Institute Of Technology | Modulator for frequency-shift keying of optical signals |
US10162111B1 (en) * | 2018-05-31 | 2018-12-25 | Lightwave Logic Inc. | Multi-fiber/port hermetic capsule sealed by metallization and method |
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JP2002054998A (en) * | 2000-08-10 | 2002-02-20 | Agilent Technol Inc | Optical sampling system |
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US4936645A (en) | 1989-08-24 | 1990-06-26 | Hoechst Celanese Corp. | Waveguide electrooptic light modulator with low optical loss |
GB2322205B (en) | 1997-11-29 | 1998-12-30 | Bookham Technology Ltd | Stray light absorption in integrated optical circuit |
US6181456B1 (en) | 1999-04-01 | 2001-01-30 | Uniphase Telecommunications Products, Inc. | Method and apparatus for stable control of electrooptic devices |
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2002
- 2002-02-12 US US10/076,020 patent/US6647185B2/en not_active Expired - Lifetime
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WO2008137204A1 (en) * | 2007-03-20 | 2008-11-13 | Massachusetts Institute Of Technology | Modulator for frequency-shift keying of optical signals |
US20100104277A1 (en) * | 2007-03-20 | 2010-04-29 | Massachusetts Institute Of Technology | Modulator for frequency-shift keying of optical signals |
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